In the manufacture of circuit boards it is known to use electroplating to provide the circuits. In the process, a substrate has bonded to its surface a thin film or foil, usually of copper. Then by a photolithographic or other process, a non-conductive layer is formed on the copper film in a pattern which may be termed the negative of the electrical circuit to be formed on the substrate; that is, the openings in the non-conductive pattern leave exposed the copper foil in a pattern which is to be that of the conductive circuit. The areas covered by the non-conductive pattern are eventually the non-conductive part of the final board. After building up a layer of electroplated metal on the exposed foil, the non-conductive pattern is removed or dissolved, and the exposed copper underneath is etched away, leaving the layer of electroplated metal on what remains of the copper film.
The current tendency to increase the density and the complexity of electrical circuits generally requires finer lines and spacing on the circuit boards than before. In order to achieve finer lines, thinner foils are applied to the substrate, in order to reduce plating times and also to reduce the undesired effects of undercutting when the circuit is etched.
The use of thinner foils creates an undesired effect during the plating process. When a circuit board is clamped at the top to form an electrical connection, the thinner foil introduces a voltage drop across the foil from top to bottom, i.e. the bottom is more positive than the top which is connected to the negative terminal of the source. This diversity in voltage causes an increase in current density distribution, causing greater current density at the higher voltage portions of the foil, near the bottom. Variations in current density result in undesired variations in thickness of the current carrying final circuit plated elements. Excess plating is required at some parts to reach the minimum plating thickness called for overall. Uniformity is desirable for several reasons. For example, to use the shortest plating time consistent with providing the minimum acceptable thickness at any point in the circuit, to ease monitoring of plated thickness, and to reduce the use of plating metal.
Further, in plating circuit boards, one or more forms of agitation are employed to prevent metal ions from being depleted at the cathode surface. Mechanical motion of the cathode is one form, "mechanical agitation". Another is to place a sparger, a tube with holes perforated along the length, under the cathode. Air forced into and emerging from the sparger displaces and agitates the electrolyte and causes fresh electrolyte to come into contact with the cathode. This method is called "air agitation". A third method, "solution agitation" is to use a sparger and pump electrolyte, rather than air, into it and direct the flow from the sparger towards the cathode from openings in the sparger. "Solution agitation" also tends to improve the uniformity of plating and avoids the exhaustion of the electrolyte at the agitated portion of the cathode.
In U.S. Pat. No. 3,743,583 to Castonguay, 1973 for Printed Circuit Board Fabrication, is described a tunnel between anode and cathode intended to prevent diffusion in the bath, and thus to deposit slowly and uniformly on the printed circuit board.
The U.S. Pat. No. 3,809,642 to Bond, et al 1974 for Electroforming Apparatus Including An Anode Housing With A Perforate Area For Directing Ion Flow Towards The Cathode, describes an apparatus which uses a pair of telescoped tubes having apertures directed toward the cathode, for directing flow to obtain precise electrodeposition. Bond, et al thus describes solution agitation.
U.S. Pat. No. 3,962,047 to Wagner, 1976 for Method For Selectively Controlling Plating Thicknesses, discloses a shield for plating a greater thickness of metal on one side of a member than on the other side.
U.S. Pat. No. 4,304,641 to Grandia, et al 1981, discloses a rotary electro-plating cell in which the differential flow distribution of electrolyte, and thus a controlled current distribution is obtained by the difference in size and spacing of the nozzles.